In a magnetic recording-reproduction apparatus of the helical scanning method, which uses a rotary magnetic head, recorded tracks are formed during recording with a tilt at a predetermined angle to the direction of the motion of a magnetic tape. In particular, in a magnetic recording-reproduction apparatus that records and reproduces a digitized signal by using a plurality of magnetic heads, the effective contact angle of the magnetic tape with respect to a rotary drum is set to be smaller than the angle that is made by each adjacent pair of the magnetic heads. With this arrangement, it is possible to record the digitized signal discontinuously in time.
FIG. 22 illustrates an example of the installation of two magnetic heads, while FIG. 23 illustrates that of the installation of three magnetic heads. Here, in FIGS. 22 and 23, the rotation direction of a rotary drum 31 is indicated by an arrow X, and the direction of the movement of a magnetic tape 32 is indicated by an arrow Y.
In FIG. 22, angle D1 that is made by magnetic head 20a and magnetic head 20b is 180 degrees, and an effective contact angle D2 that is made by the magnetic tape 32 with the rotary drum 31 is set to be smaller than angle D1 by angle D3.
In FIG. 23, angle D1, which is made by each adjacent pair of magnetic heads among three magnetic heads 21a, 21b and 21c is 120 degrees, and as with the above case, an effective contact angle D2, which is made by the magnetic tape 32 with the rotary drum 31, is set to be smaller than angle D1 by angle D3.
Further, FIG. 24 illustrates an example of the multichannel system wherein two sets of magnetic heads, each set having a pair of magnetic heads, are installed in a rotary drum.
In FIG. 24, angle D1, which is made by one set of the magnetic heads 22a, 22b and another set of the magnetic heads 23a, 23b, is 180 degrees, and an effective contact angle D2, which is made by the magnetic tape 32 with the rotary drum 31, is set to be smaller than D1 by angle D3.
Here, the magnetic recording-reproduction apparatus, which uses a rotary magnetic head, generally has a problem of track vacillation. Track vacillation is a phenomenon where the center of an actually recorded track deviates off the center of the original straight track. The occurrence of track vacillation is caused by variation of tension applied to a magnetic tape in motion and adverse effects of mechanical vibration.
For this reason, it is necessary to provide a tracking control in order to allow the rotary magnetic head to follow track vacillation and to improve the quality of the reproduced signal. Moreover, in the case of varied speed reproduction such as slow motion reproduction and still reproduction, wherein the running speed of the magnetic tape is varied from that used during recording, a high-quality reproduced signal with less picture blurring and less guard-band noise can be obtained by allowing the rotary magnetic head to follow a recorded track accurately.
One method for providing such a tracking control as described above, which is known to the art, is a head moving mechanism wherein the rotary magnetic head is shifted in the width direction of a recording track according to a controlling signal. Here, simultaneously as a recording track is reproduced, a signal is obtained by detecting a relative position between the recording track and the rotary magnetic head, and this signal is used as the above-mentioned controlling signal. Further, in order to form recorded tracks having a constant pitch, the rotary magnetic head is kept at a predetermined level during recording.
FIGS. 25 and 26 show one example of the above-mentioned head moving mechanism.
FIG. 25 is a plan view of a rotary-magnetic-head dynamic track following device having the conventional head moving mechanism. FIG. 26 is a vertical sectional view taken along the line XI--XI of FIG. 25.
A rotary-magnetic-head dynamic track following device 40 is constituted of a head moving mechanism 50, a rotary drum 31, a fixed drum 33, a motor section 51, a bearing 52 integral with a shaft, and a rotary transformer 53.
A shaft-receiving hole 33a is formed in the center of the upper surface of the fixed drum 33. The shaft of the bearing 52 is inserted through the shaft-receiving hole 33a. The bearing 52 integral with the shaft is constituted of a shaft 52a, a pair of upper and lower outer rings 52b, a pair of upper and lower balls 52c, a collar 52d, and a pre-load spring 52e. The upper portion of the cylinder-like collar 52d is inserted into the shaft-receiving hole 33a so as to fit with the inner wall thereof.
The pair of upper and lower outer rings 52b are respectively fixed to the upper side and lower side of the collar 52d. The pre-load spring 52e is fitted between these outer rings 52b. Along the inner wall of each outer ring 52b are disposed a plurality of the balls 52c, which support the shaft 52a. The shaft 52a is capable of rotating smoothly about its center line as an axis of rotation.
The motor section 51 is installed at the lower side of the fixed drum 33. The motor section 51 is constituted of a motor stator 51a, a motor rotor 51b, and a collar 51c. The cylinder-like motor stator 51a is secured to the lower side of the fixed drum 33. The collar 51c is fixed to the lower side of the shaft 52a that is protruding from the lower surface of the fixed drum 33. The motor rotor 51b, which is coupled to the lower surface of the collar 51c, is aligned face to face with the motor stator 51a.
The cylinder-like rotary drum 31 is installed on the upper side of the fixed drum 33. The rotary drum 31 is fixed to the shaft 52a that is protruding from the upper surface of the fixed drum 33. The rotary drum 31 is provided with a plurality of cylindrical cavity sections having openings at the lower surface of the rotary drum 31. The cylindrical cavity sections (which are indicated by broken lines in FIG. 25) are formed symmetrically with respect to the axis of rotation of the rotary drum 31. Further, openings 31a are formed in the upper surface of the rotary drum 31. Each of the openings 31a provides a path through each cylindrical cavity section.
A part of the head moving mechanism 50 that is installed inside the cylindrical cavity section is exposed through the opening 31a. Further, a position sensor 34 is disposed above the opening 31a with a predetermined space in between. Then, a slip ring 35 is securely fixed to the upper surface of the rotary drum 31.
The rotary transformer 53 is constituted of a ring-shaped rotor transformer 53a and a stator transformer 53b. The stator transformer 53b is fixed to a ring-shaped groove formed on the upper surface of the fixed drum 33. The rotor transformer 53a is fixed to the bottom surface of the rotary drum 31. The rotor transformer 53a and the stator transformer 53b are aligned face to face with each other having a slight space in between.
A brush 36b, which is supported by a support member 36a, is arranged to contact the slip ring 35 of the rotary-magnetic-head dynamic track following device 40 that is arranged in a manner as described above. That is, the brush 36b and the slip ring 35 are electrically connected to each other.
Next, an explanation will be given on the arrangement of the head moving mechanism 50 by reference to FIG. 27, which shows an enlarged drawing of the head moving mechanism 50 in FIG. 26. In FIG. 27, the head moving mechanism 50 is constituted of a yoke section, a head movable section, and a permanent magnet.
The yoke section is constituted of an upper surface disc yoke 54a, a bottom surface disc yoke 54b, and a cylindrical yoke 54c. The cylindrical yoke 54c is inserted into the cylindrical cavity section formed in the rotary drum 31. The upper surface disc yoke 54a is fixed to the upper surface of the cylindrical yoke 54c and the bottom surface disc yoke 54b is fixed to the bottom surface of the cylindrical yoke 54c. A cut-out window is formed in the lower side of the cylindrical yoke 54c. Further, a hole is formed through the upper surface disc yoke 54a. The hole matches the opening 31a that is formed in the rotary drum 31.
Thus, the yoke section, which is empty inside, has a cylinder shape with the cut-out window in one portion thereof. Permanent magnets 55 and a head movable section are installed in the empty section inside the yoke section. The column-shaped permanent magnets 55 respectively secured to the upper surface disc yoke 54a and the bottom surface disc yoke 54b are aligned face to face with each other having a predetermined space in between. Here, since the yoke section is made of a material that allows magnetic fluxes to pass therethrough, a magnetic flux, which are caused by the head moving mechanism, form a closed loop in the yoke section.
The head movable section is constituted of an insulator 50a, a coil 50b, an upper support spring 50c, a lower support spring 50d, and a magnetic head 20.
Here, the upper support spring 50c and the lower support spring 50d are respectively fixed to the upper and the lower portions of the cylindrical yoke 54c. The upper support spring 50c and the lower support spring 50d are capable of moving in the directions indicated by arrows P in FIG. 27.
The magnetic head 20, which is secured to the lower support spring 50d, is exposed out of the rotary drum 31 along its outer circumferential face through the cut-out window formed in the cylindrical yoke 54c and the rotary drum 31. The cylindrical insulator 50d, whereon the coil 50b is wound, is secured to the upper support spring 50c in its upper end face, and to the lower support spring 50d in its lower end face. The two permanent magnets 55 facing each other are inserted into the insulator 50a.
A plurality of the head moving mechanisms 50, arranged as described above, are installed in the rotary drum 31. That is, in the cases of FIGS. 22 and 24, two head moving mechanisms 50 are installed in the rotary drum 31, while in the case of FIG. 23, three of them are installed therein.
Next, an explanation will be given on the operation of the conventional rotary-magnetic-head dynamic track following device provided with the above-mentioned head moving mechanism 50 by reference to FIGS. 26 and 27.
First, the motor section 51 is activated. In other words, the motor rotor 51b rotates with respect to the motor stator 51a. The torque of the motor rotor 51b is transmitted to the rotary drum 31 through the shaft 52a. Thus, the rotary drum 31 rotates. As the rotary drum 31 rotates, the opening 31a of the rotary drum 31 passes below the position sensor 34. During the passage of the opening 31a, the position of the upper support spring 50c along the upward or downward arrow P is detected by the position sensor 34.
According to the detection, a controlling current is supplied to the coil 50b through the brush 36b and the slip ring 35. In response to the current flowing in the wiring 50b and the magnetic flux exerted from the permanent magnets 55, the insulator 50a, whereon the coil 50b is wound, is subjected to a force exerted in the direction along either the upward or downward arrow P. Since the upper support spring 50c and the lower support spring 50d, both supporting the insulator 50a, are capable of moving in the direction along either the upward or downward arrow P, the insulator 50a is shifted in the direction toward which the force is exerted. When the insulator 50a is thus shifted, the magnetic head 20, which is securely fixed to the lower support spring 50d, is also shifted in the same arrow P direction. Thus, the magnetic head 20 is shifted to a desired position.
A reproduced signal from the magnetic head 20 is sent to a peripheral device through the rotor transformer 53a and the stator transformer 53b. Further, a recording signal is supplied to the magnetic head 20 from a peripheral device through the stator transformer 53b and the rotor transformer 53a.
As another example of such a head moving mechanism, the following description will discuss a rotary magnetic head apparatus that is disclosed in Japanese Examined Patent Publication 61-55173 (55173/1986) by reference to FIG. 28.
The magnetic head 4 is located in a space between the upper rotary drum 1 and the lower fixed drum 13. The rotary drum 1 and the magnetic head 4 are integrally rotated by a rotation supporting mechanism fixed to a rotation axis 15. The rotation supporting mechanism is constituted of a bottom support member 9, a center pole 12, a permanent magnet 8, a yoke 7, a spring fixing member 6, a plate spring member 5, a bobbin 10, and a magnetic head support plate 3, which are all located in this order from the rotation axis 15.
The magnetic head support plate 3 has a disc shape or a rod shape, and is fixed to the upper end of the small cylinder section of the bobbin 10. Ends of a plurality of the plate spring members 5 are respectively fixed to the side face of the small cylinder section and the spring fixing member 6, and the magnetic head 4 is thus supported at a predetermined level with respect to the bottom support member 9. A coil 11 is wound around the side face of the large cylinder section of the bobbin 10. Here, the bobbin 10 is made of an insulating member.
Here, the permanent magnet 8, the yoke 7, a cavity section 16 and the center pole 12 are arranged to form a magnetic circuit. The coil 11 is disposed in the cavity section 16; therefore, when the controlling signal is supplied to the coil 11, a driving force is generated by the interaction between the current and the magnetic field in such a manner that the bobbin 10 is shifted in the axis direction of the rotation axis 15. With this arrangement, the magnetic head 4 is shifted to a position at which the driving force balances the elastic force of the plate spring member 5.
In addition, the controlling signal is supplied to the coil 11 from a brush fixed outside (not shown) through the slip ring 14. Further, the rotary drum 1 is fixed to an axis portion that is secured to the center of the center pole 12 through the disc 2.
In the case where a magnetic recording-reproduction apparatus, which is provided with a rotary magnetic head, is used in a highly humid atmosphere or under an environmental condition having abrupt temperature changes, minute drops of water are formed between the magnetic tape and the lower drum, resulting in a thin water screen. The viscous resistance caused by the existence of this water screen tends to make the magnetic tape entirely adhere to the lower drum, or in the case of the intermittent adhesion, tends to cause a sort of self-excited vibration in the tape.
In order to achieve compactness and light weight of such a magnetic recording-reproduction apparatus as described above, the magnetic recording density of the magnetic tape needs to be improved. For this reason, metal tapes constituted of a resin base whereon a nickel-cobalt alloy is deposited or applied have come to be employed as those magnetic tapes. However, in this case, the metal tape and the lower drum, both of which are metals, are brought to contact and slide on each other. The result is that the relative friction coefficient becomes greater, thereby making it difficult to stabilize the travel of the magnetic tape.
In order to solve these problems, for example, such a magnetic recording-reproduction apparatus that is disclosed in Japanese Examined Patent Publication No. 60-19061 (Tokukoshou 19061/1985) is provided with spiral grooves 60 formed on the circumferential edge of the bottom surface of an upper drum 62, as is illustrated in FIGS. 29(a) and (b). Here, portions between the spiral grooves 60 are referred to as lands 61; these lands 61 are then aligned face to face with the upper surface of the lower drum with a minute gap in between. In this arrangement, since the upper drum 62 has a relative speed in the rotation direction indicated by an arrow R in FIG. 29(a) with respect to the lower drum, the spiral grooves 60 generate a dynamic pressure that is exerted from the rotation center O toward the periphery of the upper drum 62.
The dynamic pressure is increased or reduced depending on the width H of the circumferential edge of the bottom surface whereon the spiral grooves 60 are provided. More specifically, if the width H is increased by widening the difference between the distance from the rotation center O to the outer circumferential edge of the upper drum 62 and that from the rotation center O to the inner edge of the bottom surface, a greater dynamic pressure is generated. This dynamic pressure makes the magnetic tape slightly float from the circumferential face of the lower drum, thereby reducing the load imposed on the tape in travel.
Here, the following various problems are presented in the above-mentioned prior art devices.
For example, in the rotary-magnetic-head dynamic track following device 40, if the diameter and height of the rotary-magnetic-head dynamic track following device 40 are shortened in order to make the magnetic recording-reproduction apparatus compact and light, the diameter and height of the head moving mechanism 50 also need to be shortened in accordance with this modification. In this case, the amount of shift of the magnetic head 20 in relation to the current supplied to the coil 50b, the so-called current sensitivity, is reduced. This lowering of the current sensitivity is caused by a lowering of the magnetic property due to the miniaturization of the permanent magnet 55 and a rising of the magnetic resistance due to the shortage of the overlapping area between the inner wall of the cylinder yoke 54c and the circumferential face of the permanent magnet 55.
In order to compensate for this reduction, two methods are suggested: (i) a method for lowering the spring constants of the upper support spring 50c and the lower support spring 50d so as to obtain a predetermined amount of shift and (ii) a method for increasing the current to be supplied to the coil 50b.
However, in the case of the former compensation method, wherein the rigidities of the upper support spring 50c and the lower support spring 50d are lowered, the resonance frequency of the rotary-magnetic-head dynamic track following device 40 becomes lowered, and its response characteristic deteriorates. As a result, due to disturbances caused by varied tape tension that are imposed on the lower support spring 50d through the magnetic head 20, problems arise in which vacillation of the recorded tracks becomes larger with the result that the magnetic head cannot follow the recorded tracks during reproduction (runout in tracking).
In the case of the latter compensation method, the same problems as described above arise due to thermal distortion of the head moving mechanism 50, which is caused by the temperature increase that results from heat generation of the coil 50b; furthermore, a problem arise wherein power consumption of the rotary-magnetic- head dynamic track following device 40 increases. The above-mentioned problems become more serious when special provisions are made to make the magnetic recording-reproduction apparatus compacter and lighter with a view to enhancing the recording density by shortening the track pitch.
Moreover, in the conventional rotary magnetic head apparatus as disclosed in Japanese Examined Patent Publication No. 61-55173 (55173/ 1986), a motor section for rotating the rotation axis 15 and a rotation transformer section for transmitting a recording signal to the magnetic head 4 and a reproduced signal from the magnetic head 4 in a non-contact state need to be added thereto; this makes the entire rotary magnetic head apparatus bulky. As a result, it becomes difficult to provide a compact and light-weight magnetic recording-reproduction apparatus in which the rotary magnetic head apparatus is installed; therefore, it becomes difficult to achieve a technological development to provide high-density recording by the use of a compacter cassette tape in the compact and light-weight magnetic recording-reproduction apparatus.
Furthermore, in the arrangement wherein the spiral grooves 60, such as disclosed in Japanese Examined Patent Publication No. 60-19061 (Tokukoshou 19061/1985), are provided on the upper drum; if the thickness (width H) of the circumferential edge of the bottom surface of the upper drum is decreased in order to achieve compactness and light weight, it will be difficult to obtain a sufficient effect for reduction of the load imposed on the tape in travel. This is because, if the width H of the circumferential edge of the bottom surface is decreased, the dynamic pressure to be generated will also be reduced; thus, the amount of floatation of the magnetic tape with respect to the circumferential face of the lower drum is reduced.
If the thickness of the circumferential edge of the bottom surface is increased with the upper drum being kept compact in order to obtain a desired dynamic pressure, the head moving mechanism that is to be installed in the upper drum needs to be made compact. As a result, the same problems as described in the above rotary-magnetic-head dynamic track following device 40, such as lowering of the magnetic characteristic due to the shortage of the volume of the permanent magnet, increased power consumption of the rotary-magnetic-head dynamic track following device, deterioration of the response characteristic of the head moving mechanism, etc. are presented.